This invention relates to a flexible circuit board which includes an installation portion for installing electronic parts and a flexible portion provided adjacent thereto; and a method of making the same. More particularly, this invention relates to a novel flexible circuit board wherein a desired electrolytic plating is provided over both the installation portion and an adjacent transition portion located between the installation portion and the flexible portion of the circuit board; and a method of making the same.
Flexible circuit boards are presently used in many areas as an effective circuit-providing means in various electrical and electronic instruments. One type of flexible circuit board is known which includes an installation portion on which electronic parts can be installed, and a flexible portion provided adjacent to the installation portion which does not have a parts installing capability. The general prior art technique used for manufacturing such a flexible circuit board having an installation portion and a flexible portion comprises initially preparing a flexible circuit board material by coating an electro-conductive foil, such as copper foil or the like, on both sides of a suitable flexible insulating base material. Next, through-holes for electrical connection are drilled at required locations on the material. A nonelectrolytic chemical plating layer is then provided over the entire board starting material so as to obtain an electrical connection through the through-holes. This nonelectrolytic plating step is followed by an electrolytic plating step so as to form an electrolytic plating layer over the nonelectrolytic chemical plating layer and thereby secure the electrical connection through said through-holes. Thereafter, desired circuit patterns are formed by an etching process, whereupon first circuit patterns having through-hole conductive parts are formed on both sides of the installation portion and a second circuit pattern adjoining one or both of the first circuit patterns is formed on one or both sides of the flexible portion provided adjacent to the installation portion.
In the above-described known technique for making such flexible circuit boards, an electrolytic plating having a hard or rigid texture is provided over the entire circuit pattern of the installation and the flexible portions; and consequently the flexibility of the flexible portion is reduced to a large extent. Moreover, a serious problem arises in that bending or flexing motion causes breakage of the circuit pattern at the location of the bending motion. As a means of eliminating this kind of problem at the flexible portion of the circuit board, a technique is known wherein a circuit pattern is provided on only one side of the flexible portion; and no plating layer is provided over that circuit pattern. Consequently, the flexibility of the flexible part is maintained and circuit breakage is prevented. See, for example, Japanese Utility Model Sho 56-54607 (1981) and Japan Patent Application Sho 57-79697 (1982).
In the above-mentioned prior art structure, wherein no plating layer is provided over the circuit pattern of the flexible portion, an electrolytic plating layer is coated over the conductive foil region of the installation portion, (whereas the flexible portion has the initial conductive foil alone). This results in a discontinuity forming at the boundary between the two portions. When circuit patterns are prepared under such a condition (by utilizing conventional photoresist and etching processes), it is difficult to form a circuit pattern accurately in the vicinity of the discontinuity portion due to the effect of the discontinuity. Accordingly, a problem develops in that the risk of circuit breakage at the discontinuity portion becomes higher due to stress concentration. Moreover, while it is necessary to mask the flexible portion in order to provide a nonelectrolytic chemical plating and an electrolytic plating for the installation portion alone, this masking process leads to complications of the overall process because the masking process requires a large number of steps; also the mask must be removed at a later stage of the process.